On a conventional turret punch press, a die is arranged on the lower turret via a die holder and dies are at each die holder. The mounting of the die holder on the lower turret is carried out by the coupling of one end to a pin projecting from the lower turret and fixing the other end with a bolt.
Having a conventional arrangement as described above where the die holder is fixed by a bolt necessitates space in the inner diameter of the bolt hole and because of this, there are limits to the accuracy of the mounting of the die holder on the lower turret. As a result, core dis-alignment between the die and punch is generated and it is necessary to arrange enough clearance between the die and punch in order to be able to absorb this core dis-alignment.
However, when the clearance between the die and punch increases, the barb generated on the reverse side of the work punch hole exponentially increases and clean punch processing becomes difficult.
Apart from the accuracy of the mounting of the die holder, rigidity and heat deformation of the frame as well as core dis-alignment of the upper and lower turrets have been posited as reasons for the generation of core dis-alignment between the die and punch and in order to limit the generation of barb, it is necessary to generally improve these. However, accuracy of the mounting of the die holder is a large cause of core dis-alignment.